The present invention relates generally to medical devices and more particularly to a method of manufacturing a self-expanding stent-graft with wrinkles in the expanded relaxed state.
Stents have become relatively common devices for treating a number of organs, such as the vascular system, colon, biliary tract, urinary tract, esophagus, trachea and the like. Stents are useful in treating various ailments including blockages, occlusions, narrowing conditions and other related problems that restrict flow through a passageway (generally referred to as a stenosis). Stents are also useful in a variety of other medical procedures including treating various types of aneurysms.
For example, stents may be used to treat numerous vessels in the vascular system, including coronary arteries, peripheral arteries (e.g., carotid, brachial, renal, iliac and femoral), and other vessels. Stents have become a common alternative for treating vascular conditions because stenting procedures are considerably less invasive than other alternatives. As an example, stenoses in the coronary arteries have traditionally been treated with bypass surgery. In general, bypass surgery involves splitting the chest bone to open the chest cavity and grafting a replacement vessel onto the heart to bypass the stenosed artery. However, coronary bypass surgery is a very invasive procedure that is risky and requires a long recovery time for the patient. By contrast, stenting procedures are performed transluminally and do not require open surgery. Thus, recovery time is reduced and the risks of surgery are minimized.
Many different types of stents and stenting procedures are possible. In general, however, stents are typically designed as tubular support structures that may be inserted percutaneously and transluminally through a body passageway. Typically, stents are made from a structure that wraps around at least a portion of a circumference and are adapted to compress and expand between a smaller and larger diameter. Stents may be self-expanding so that they elastically expand out to the larger diameter, or may be balloon-expandable so that they require a force to expand to the larger diameter. However, other types of stents are designed to have a fixed diameter and are not generally compressible. Although stents may be made from many types of materials, including non-metallic materials and natural tissues, common examples of metallic materials that may be used to make stents include stainless steel and nitinol. Other materials may also be used, such as cobalt-chrome alloys, amorphous metals, tantalum, platinum, gold, titanium, polymers and/or compatible tissues. Typically, stents are implanted within an artery or other passageway by positioning the stent within the lumen to be treated and then expanding the stent from a compressed diameter to an expanded diameter. The ability of the stent to expand from a compressed diameter makes it possible to thread the stent through narrow, tortuous passageways to the area to be treated while the stent is in a relatively small, compressed diameter. Once the stent has been positioned and expanded at the area to be treated, the tubular support structure of the stent contacts and radially supports the inner wall of the passageway. The implanted stent may be used to mechanically prevent the passageway from closing in order to keep the passageway open to facilitate fluid flow through the passageway. Conversely, stents may also be used to support a graft layer to prevent fluid flow through the side walls of the stent. However, these are only some of the examples of how stents may be used, and stents may be used for other purposes as well.
Self-expanding stents are one common type of stent used in medical procedures. Self-expanding stents are increasingly being used by physicians because of their adaptability to a variety of different conditions and procedures. Self-expanding stents are usually made of shape memory materials or other elastic materials that act like a spring. Typical metals used in this type of stent include nitinol and 304 stainless steel. However, other materials may also be used. To facilitate stent implantation, self-expanding stents are normally installed on the end of a catheter in a low profile, compressed state. The stent is typically retained in the compressed state by inserting the stent into a sheath at the end of the catheter. The stent is then guided to the portion of the vessel to be treated. Once the catheter and stent are positioned adjacent the portion to be treated, the stent is released by pulling, or withdrawing, the sheath rearward. Normally, a step or other feature is provided on the catheter to prevent the stent from moving rearward with the sheath. After the stent is released from the retaining sheath, the stent springs radially outward to an expanded diameter until the stent contacts and presses against the vessel wall. Traditionally, self-expanding stents have been used in a number of peripheral arteries in the vascular system due to the elastic characteristic of these stents. One advantage of self-expanding stents for peripheral arteries is that traumas from external sources do not permanently deform the stent. As a result, the stent may temporarily deform during unusually harsh traumas and spring back to its expanded state once the trauma is relieved. However, self-expanding stents may be used in many other applications as well.
Stents may also be used in combination with other components to treat a number of medical conditions. For example, stent-graft assemblies are commonly used in the treatment of aneurysms. As those in the art well know, an aneurysm is an abnormal widening or ballooning of a portion of an artery. Generally, this condition is caused by a weakness in the blood vessel wall. High blood pressure and atherosclerotic disease may also contribute to the formation of aneurysms. Common types of aneurysms include aortic aneurysms, cerebral aneurysms, popliteal artery aneurysms, mesenteric artery aneurysms, and splenic artery aneurysms. However, it is also possible for aneurysms to form in blood vessels throughout the vasculature. If not treated, an aneurysm may eventually rupture, resulting in internal hemorrhaging. In many cases, the internal bleeding may be so massive that a patient can die within minutes of an aneurysm rupture. For example, in the case of aortic aneurysms, the survival rate after a rupture can be as low as 20%.
Traditionally, aneurysms have been treated with surgery. For example, in the case of an abdominal aortic aneurysm, the abdomen is surgically opened, and the widened section of the aorta is typically dissected longitudinally. A graft material, such as Dacron, is then inserted into the vessel and sutured at each end to the inner wall of the non-widened portions of the vessel. The dissected edges of the vessel may then be overlapped and sutured to enclose the graft material within the vessel. In smaller vessels where the aneurysm forms a balloon-like bulge with a narrow neck connecting the aneurysm to the vessel, the surgeon may put a clip on the blood vessel wall at the neck of the aneurysm between the aneurysm and the primary passageway of the vessel. The clip then prevents blood flow from the vessel from entering the aneurysm.
An alternative to traditional surgery is endovascular treatment of the blood vessel with a stent-graft. This alternative involves implanting a stent-graft in the blood vessel across the aneurysm using conventional catheter-based placement techniques. The stent-graft treats the aneurysm by sealing the wall of the blood vessel with a generally impermeable graft material. Thus, the aneurysm is sealed off and blood flow is kept within the primary passageway of the blood vessel. Increasingly, treatments using stent-grafts are becoming preferred since the procedure results in less trauma and a faster recuperation.
One problem that can occur with self-expanding stent-grafts is that the graft layer can become excessively tight on the stent when the stent is in its expanded relaxed state. This can be undesirable because it may overstress the graft layer and raise fatigue concerns. This may be a particular concern with stent-grafts that are implanted in passageways that are subject to a high degree of axial stretching, bending, twisting or changes in radial shape. One example of a passageway that undergoes an extreme amount of such movement is the superficial femoral artery.
Although overstretching of the graft layer may be a concern with many types of graft materials, inelastic graft materials may raise particular concerns. For example, Dacron (polyethylene terephthalate or PET) is one graft material that is commonly preferred due to its low permeability and biocompatibility. However, it is possible for Dacron to crack if excessive tensile forces are applied to the material.
Accordingly, the inventors believe a method for making a stent-graft that minimizes overstretching of the graft layer may be desirable.